Add the initial support for Elgin R1 board, which is based on the
RV1108 SoC and has the following features currently supported in
U-Boot:

- UART
- eMMC
- USB

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>